This invention relates to an image-receiving sheet, and more particularly to an image-receiving sheet, for use in electrophotographic system, thermal transfer system or the like where the image-receiving sheet is heated at the time of image formation, that enables an opaque detection mark to be rendered transparent upon exposure to heat and pressure at the time of image formation.
In recent years, formation of a monochromatic image of black as well as formation of a full-color image by mixing of three-color toners of yellow, magenta, and cyan, or four-color toners of the above three colors, yellow, magenta, and cyan, and black using an electrophotographic system has been put to practical use.
In order to steadily record and hold record information, such as letters and images, an image-receiving sheet used in the electrophotographic copying machine comprises a substrate having thereon a receptive layer. This image-receiving sheet is used, for example, for OHP (overhead projector) as information transmission means which is used in lecture meetings, schools, enterprises, other briefing sessions, exhibitions and the like.
In the electrophotographic system, detection means is provided in order to detect paper jamming or to discriminate one side of paper from the other side, or, in the case of color image formation, to register image patterns of three colors of yellow, magenta, and cyan, or four colors of the above three colors, yellow, magenta, and cyan, and black on an identical image-receiving sheet. In particular, in image formation on a transparent image-receiving sheet, for example, optical detection means is provided wherein light from a light source is applied to a transparent image-receiving sheet and the presence or absence of reflected light or the presence or absence of transmitted light is detected by a photosensor.
For example, a detection mark has been formed using an opaque ink or an ink having metallic luster. Further, the applicant of the present invention has proposed, in Japanese Patent Laid-Open Nos. 49581/1995 and 56376/1995, an image-receiving sheet having an opaque detection mark that can be rendered transparent upon heating at the time of image formation and, when the formed image is projected by an overhead projector, permits only a necessary image to be projected without creation of a visually noticeable shadow in the projection of the detection mark.
The detection mark described in Japanese Patent Laid-Open Publication Nos. 49581/1995 and 56376/1995 is formed by coating a transparent resin varnish comprising a resin, a low-boiling good solvent, and a high-boiling poor solvent and then drying the coating to cause gelation, thereby forming an opaque porous layer. This detection mark, when heated at the time of image formation, permits the resin to be melted and rendered transparent, thus erasing the detection mark.
In the above conventional image-receiving sheets, the capability of the detection mark to be rendered transparent upon heating at the time of image formation and the coating strength and the like of the detection mark layer vary depending upon the resin constituting the detection mark, that is, the type of the resin and the difference in degree of polymerization or molecular weight even in the same type of resin.
Further, in copying machines and printers for image formation in an electrophotographic system, a thermal transfer system or the like, the image-receiving sheet has become carried at a high speed in order to improve the processing efficiency. This has led to lowered energy in heating and pressing at the time of image formation.
Under the above circumstance, application of satisfactory heat and pressure to the detection mark in copying machines and printers in a high-speed electrophotographic system or thermal transfer system has become impossible, posing a problem that the conventional image-receiving sheet is unsatisfactory in the erasability of the detection mark upon heating at the time of image formation.
In order to solve this problem, an attempt has been made to reduce the coating thickness of the detection mark (not more than 1.7 g/m.sup.2 on dry weight basis) to improve the erasability of the detection mark. The reduction in the coating thickness of the detection mark leads to a problem that the initial properties such as light transmittance and reflectance properties of the detection mark (i.e., properties before thermal printing at the time of image formation) are lowered where by the detection mark becomes hard to be detected.
Accordingly, an object of the present invention is to solve the above problems of the prior art and to provide an image-receiving sheet having an opaque detection mark that, when an image is formed by copying machines or printers in high-speed electrophotographic system, thermal transfer system or the like, can be rendered transparent upon exposure to heat and pressure at the time of image formation to such an extent that the detection mark is no longer visually noticeable, and, before image formation, does not undergo a lowering in infrared transmittance and reflectance properties before heating at the time of image formation.